As a typical one of the conventional liquid spraying apparatuses, an ink-jet printer is known in which ink is sprayed onto recording paper from tiny holes in a liquid-spraying head to form graphics and/or characters on the paper. The ink-jet printer is advantageous because of its low running cost, compact design and easy printing of an image in colors. In the inkjet printer, colored ink is supplied from an ink cartridge to an ink well or the like in a liquid-spraying head. An ink cartridge is provided for each ink color such as yellow, magenta, cyan or black, for example.
In the printer of this type, ink supplied to the ink well or the like is pressurized in the ink well by a pressure-producing element or the like provided in the ink well for spraying from tiny holes, namely, so-called nozzles, formed in the liquid-spraying head.
More particularly, the ink in the ink well is heated by a resistance heater provided in the ink well to produce bubbles in the ink on the resistance heater, and the ink is sprayed from the nozzles under a pressure produced by the bubbles inside the ink well for projection onto recording paper or the like to form graphics and/or characters on the paper.
The ink-jet printers include a serial printer in which, as the liquid-spraying head having an ink cartridge mounted thereon is moved across the width of recording paper, namely, in a direction nearly perpendicular to the moving direction of the paper, ink in a predetermined color is projected onto the paper, and also a line printer in which ink is sprayed in line from nozzles on a liquid-spraying head that have an ink-spraying range generally as wide as the width of recording paper. That is, the nozzles are arrayed across the paper width.
In the serial printer, the recording paper is kept from moving while the liquid-spraying head is moving in the direction nearly perpendicular to the moving direction of the paper, the liquid-spraying head sprays ink for projection onto the paper while moving, and these operations are repeated to form characters and/or graphics on the paper. On the other hand, in the line printer, the liquid-spraying head is immobilized or fixed to such an extent as to slightly be movable in order to prevent any uneven imprinting, and the liquid-spraying head sprays ink in line for projection onto the paper which is continuously being moved to form characters and/or graphics on the paper.
As above, different from the serial printer, the line printer has the liquid-spraying head which is not to be moved. So, it can print at a higher speed than the serial printer. Also, since in the line printer, the liquid-spraying head has not to be moved, each of the ink cartridges can be designed larger to contain an increased volume of ink. Since the liquid-spraying head is not moved, the line printer can be designed simple and the liquid-spraying head is provided integrally with each ink cartridge.
For quality printing by such an inkjet printer, it is important to print while keeping a constant distance between the nozzles and recording paper. On this account, it has been proposed as disclosed in the Japanese Patent Application Laid Open No. 90858 of 1996 to keep a constant distance between the nozzles and paper in the printer of this type by keeping the paper free from sagging with an appropriate load being given in a direction in the plane of the paper opposite to the nozzle side of the liquid-spraying head where there are provided nozzles.
The above will be explained more specifically below with reference to FIG. 1. As shown, a printer, generally indicated with a reference numeral 201, makes printing on recording paper P opposite to a nozzle side 202a of an ink-spraying head 202. For this printing, a feed roller 203 carries the paper P in the direction of arrow X in FIG. 1 to a printing position, and then a delivery roller 204 carries the printed paper P out of the printer 201. In the course of printing, the paper P is kept tight in the carrying direction by rotating the feed roller 203 at a speed of rotation about the roller axis while rotating the delivery roller 204 at a higher speed of rotation about the roller axis.
In the printer 201, when the trailing end of the recording paper P being carried in the direction of arrow X in FIG. 2 leaves a nip point of the feed roller 203, namely, a point of the paper P the feed roller 203 nips, a load the feed roller 203 slower in speed of rotation than the delivery roller 204 applies in a direction opposite to the carrying direction, that is, a load applied to the paper P in the direction of arrow Y in FIG. 2, will not act on the paper P any longer. Thus, since the load applied by the feed roller 203 to the paper P in the direction opposite to the carrying direction will not act on the paper P when the trailing end of the paper P leaves the nip point of the feed roller 203, the paper P will be carried only by the delivery roller 204 faster in speed of rotation than the feed roller 203 and hence at a higher speed in the process of printing.
Also, since the recording paper P will expand and contract in a direction in the plane thereof due to a temperature, humidity, etc. during printing, the carrying speed will be influenced by the temperature, humidity, etc. Further, since gripping or nipping of the paper P by the rollers 203 and 204 varies from one type of the paper P to another, the paper carrying speed will depend upon the paper type as well.
Thus, in the printer 201, when the trailing end of the recording paper P leaves the nip point of the feed roller 203, the carrying speed of the paper P will be increased under the influence of the printing temperature and humidity and the type of the paper P in the process of printing so that the ink will be projected to a position displaced in the carrying direction for an increment of the paper carrying speed, namely, so-called “color mis-registration” will occur, resulting in lower definition of printed characters or graphics. Also in the printer 201, since gripping or nipping of the paper P by the rollers 203 and 204 varies depending upon the type of the paper P, the paper P will be moved at a speed depending upon the type of the paper P.
More specifically, in the printer 201, a higher temperature and humidity will elongate a belt driven by a drive motor (not shown) that drives each of the rollers 203 and 204 and so the belt will have the pitch thereof increased. Thus, the belt tension is decreased, the feed pitch per tooth of pulleys of the rollers 203 and 204, on which the belt is wound, is decreased, the moving speed of the recording paper P is decreased and hence the color mis-registration will be smaller. More particularly, it is assumed here that carrying the paper P at a higher speed at normal temperature in the process of printing will cause a displacement between a position to which ink is projected from a nozzle located upstream in the moving direction of the paper P and a position to which ink is projected from a nozzle located downstream, resulting in color mis-registration of 300 μm, for example. On this assumption, when the temperature and humidity become higher, the paper P will be carried at a lower speed and hence the color mis-registration will be 250 μm, for example, which is smaller than that at normal temperature.
On the other hand, a low temperature and humidity will contract the belt whose pitch will thus be smaller. Thus, the belt tension is increased, the feed pitch per tooth of the pulleys of the rollers 203 and 204, on which the belt is wound, is increased, the recording paper P is carried at a higher speed and hence the color mis-registration will be larger. More particularly, it is also assumed here that carrying the paper P at a higher speed at normal temperature in the process of printing causes color mis-registration of 300 μm, for example. On this assumption, when the temperature and humidity become lower, the paper P will be carried at a higher speed and hence the color mis-registration will be 350 μm, for example, which is larger than that at normal temperature.
Also in the printer 201, the recording paper P of such a type as is gripped strongly by the rollers 203 and 204 will be carried at a higher speed in the process of printing, resulting in larger color mis-registration. On the other hand, the paper P of such a type as is gripped weakly by the rollers 203 and 204 will be carried at a lower speed in the processing of printing, resulting in smaller color mis-registration.
To solve the problem of such color mis-registration, it has been proposed as disclosed in the Japanese Patent Application Laid Open No. 186086 of 1993 to form the rollers 203 and 204 for carrying the recording paper P from a soft roller and hard roller, respectively, for the purpose of attenuating a shock caused when the paper P leaves the nip point.
However, use of the soft roller to attenuate the shock when the recording paper P leaves the nip point has not yet attained the solution of the problem of color mis-registration.
Also, as disclosed in the Japanese Patent Application Nos. 186086 of 1993 and 2002-225370, it has been proposed to provide a normally unused nozzle in addition to nozzles used for printing and correct color mis-registration with ink sprayed from the unused nozzle. The techniques proposed in these Published Unexamined Patent Applications are effectively usable in a serial liquid spraying apparatus but not in any line-head liquid spraying apparatus.